Exhaust gas emission from Marine diesel engine includes nitrogen, oxygen, carbon dioxide and water vapour with smaller quantities of carbon monoxide, oxides of sulphur (SOx), nitrogen partially reacted (NOx) and non combustible hydrocarbons and particulate matter (PM) including ash and soot. Increasing emissions day by day are being driven by rising demand for shipping and the associated consumption of fossil fuels. As per the sources in 1997 only, ships which are burning Marine diesel oil and heavy fuel oil are reportedly responsible for around 7 % of global NOx emission around 4% of global Sulphur Dioxide emission and 2% of global carbon dioxide emission and it is increasing day by day. As per IMO reports now Maritime transport emits around 1000 million tonnes of CO2 annually and is responsible for about 2.5% of global greenhouse gas emission and it may further increase in coming days.
How exhaust gas products are formed and in which amount …..?
- Sulphur oxides produced by the oxidation of sulphur in fuel relatively at very low temperature during combustion process of engines.
- Nitrogen oxide generated due to the oxidation of nitrogen at a very high temperature in the cylinder. Amount of NOx production depends upon the high temperature and the time stay(residence time) of that high-temperature zone.
- Carbon monoxide resulting from the incomplete combustion due to a local storage of air and the dissociation of carbon dioxide.
- Carbon dioxide is produced due to the complete oxidation of carbon in the fuel and its amount produced is directly proportional to the mass of the fuel used(1 ton of diesel fuel produced approximately 3 tons of carbon dioxide)
- Particulate matter it is a complex mixture of the inorganic and organic compound resulting from incomplete combustion partly by Lube oil, the thermal splitting of HC from the fuel, and ash of the fuel and Lube oil sulfates and water.
- Hydrocarbons created by the incomplete combustion of fuel and Lube oil and the evaporation of fuel.
Effect of engine emission on human health and environment
- NOx is believed to be carcinogenic.
- NOx and SOx contributed to photochemical smog formation and later on, they react with the water molecule present in the atmosphere or raining water and leads to acid rain. NOx produces nitric acid where SOx produces sulphurous and sulphuric acids. Acidic rain can lead to corrosion of drinking water supply system and health hazards due to dissolve metal in the system. Forest soil can also become contaminated with higher than normal level of toxic metal. Acid rain will damage historic building and monuments.
- An unpleasant order of SOx irritates the mucous membrane.
- Sulphur deposition can also lead to increased sulphate level in soil, fostering the formation of insoluble Aluminium phosphate which can cause phosphorus deficiency and groundwater acidification.
- Unburned or vaporized hydrocarbons have an unpleasant order and are partially carcinogenic and smoke forming and irritates the mucous membrane.
- Carbon monoxide is highly toxic in high concentration.
- Carbon dioxide contributes the greenhouse effect global warming and climate change and hence to changes in the earth’s atmosphere.
- Particulates, soot and smoke emission are considered a contributory factor in causing asthma allergy and various other human health problem. Some of smallest particles are more likely to penetrate deep into the human lungs and the most minute of them perhaps even moving into the bloodstream and the chemical composition of the particulates.
Regulation for controlling emission and their limits
Control of SOx emission
SOx formation started at a very low temperature and cannot be controlled by the combustion process. Amount of SOx production is proportional to the amount of sulphur in the fuel. It cannot be reduced or controlled effectively with the help of a Diesel Engine component modification or engine operational parameter improvements and optimization.
As per the engine manufacturers under similar combustion circumstances, if the fuel contains 3% sulphur around 64 kg of SOx will be generated per ton of fuel and if 1% sulphur content fuel is used, the amount of SOx production will come down by 21 kg. So fuel with very low sulphur content is the best solution for reducing SOx emission. Low sulphur fuel also contributes to reducing the Cylinder oil consumption, reduces the possibility of acidic corrosion of engine component and exhaust system and reduction in particulate matter emission.
With the help of Exhaust gas scrubber system, we can wash the exhaust gas in contact with an alkali fluid, both Rossi water and fresh water mixed with caustic soda water(Basic solution) where acid products are produced and neutralized simultaneously.
however, these Scrubbers are heavy and require significant installation space and they produce large quantities of wastewater and sludge.The water has to be properly treated before discharge into the sea and the sludge must be disposed of at reception facilities.
Controlling NOx emission
All engine have to fulfill the NOx emission limits set by IMO as a function of Engine Speed. To Ensure this IMO compliance an engine has to be certified according to the NOx technical code and delivered with an engine international air pollution prevention EIAPP certificate of compliance.
This certification process includes NOx measurement for the engine, Stamping of the component that effect NOx formation and Providing a Technical file that is delivered to the engine. This NOx technical file includes the Survey procedures, combustion affecting engine part maintenance and range of setting( including Piston cylinder cover and liner, turbocharger and charge air cooler and the fuel injection system fuel pump injection nozzle and timing Camshaft) to be adopted to ensure that NOx emission from the given engine is under reference condition of the mentioned certificate value. The ship operators must ensure that any change to NOx critical components or setting, should go for reapproval prior to installation to avoid non-compliance with engine certification.
How Nox is produced..?
- Nitrogen and oxygen are present in plenty of amount in the combustion space and during combustion high temperature is produced to facilitate the oxidation of nitrogen and hence NOx is produced.
- Dominating influence in the formation of NOx are temperature and oxygen concentration, the higher the temperature and higher the residence time at high temperature in the cylinder, the greater is the amount of NOX produce
Because of longer combustion time span, the low-speed two-stroke engine gives high NOx emission than medium and high speed four stroke engine of equivalent output.
Methods of reducing NOx
Primary NOX reduction methods:-
Methods which can directly control the NOx production in the combustion space are known as primary methods. It can be achieved by Combustion area component design modification and optimizing engine parameters with respect to emission such as valve timing fuel injection and turbocharging. NOx reduction by 30% to 60% can be achieved by this methods. The basic aim of most of these primary measures is to lower the maximum temperature in the cylinder since this result in lower NOx emission.
Water addition either directly into the cylinder or by emulsified fuel
Water in fuel can be either emulsified or it can be injected separately in combustion to reduce NOx production. Generally, 1% of water addition in fuel reduces NOx emission by 1%.
Freshwater emulsion method
In freshwater emulsion method mix the fuel with fresh water on board the ship to form an emulsion suitable for injection into the combustion chamber. In the case of heavy fuel oil up to 30% water can be emulsified into the fuel resulting in NOx reduction approximately 30%. It proves good result with electronic control engine control and injection system but comes up with some operational demerits on cam-operated engines-
- Limitation in Emulsification of water-fuel to get correct viscosity at the time of injection.
- The injection nozzle design such as hole diameter has to be adapted to the increased quantity of liquid injected.
- Allowance must be given to the fuel pump for the rated amount of fuel and water delivery in new designs, the application on the existing running engine may lead to derating of power output.
- When the engine is running without water with such a modified nozzle design both fuel consumption and component temperatures may be penalized.
Direct water injection method-
It is effective in reducing NOx by adding mass and taking out heat from the combustion process. Water and fuel are injected in a typical water to fuel ratio of 0.4- 0.7, reducing NOx production by 50% to 60% without adversely affecting the power output of engine component. In this method, water is directly injected into the combustion space either by a separate injector or a specially designed fuel injector which can handle both fuel and water alone. In specially designed fuel injector fuel is controlled in a conventional way but water injection is electronically controlled for optimizing engine efficiency and NOx production. An additional flow fuse installed on the cylinder head side act as a safety device shutting off the water flow into the cylinder if the water needle gets stuck or in the event of excessive water flow.
Water addition methods need an additional amount of fresh water storage onboard as ships with daily high fuel consumption needs a substantial quantity of water and an associated handling system.
Combustion Air humidification.
When the water vapour is mixed with the compressed Scavenge air, its increase the specific heat capacity of the mixture and replaces air which enables less available oxygen for NOx formation. High specific heat provides relatively high heat absorbing capacity and retarded temperature for the same amount of heat produced. Here water is added to the scavenge air by the special high fog nozzle and introduce water directly into the charge air stream after the turbocharger in the form of very small droplets where its evaporate very quickly in an environment of more than 200 C and high air velocity. Amount of water injected into the inlet air depends on the temperature and pressure of the mixture.
Humid air motor system (HAM)
It is introduced by MAN diesel where turbocharged scavenge air is saturated with water vapour produced aboard the ship using sea water and engine heat. Here Combustion flame front is slowed to a more uniform combustion temperature and more mass flows through the cylinders. Which gives the potential to increase the energy efficiency of the engine with less NOx production. It is having very low operating and maintenance cost and some additional advantages like –decreased Cyl lube oil consumption, cleaner combustion space with relatively fewer deposits on the turbocharger turbine and the full exhaust tract along with no significant influence on specific fuel consumption, and increase in emission of carbon monoxide, hydrocarbon and smoke.
Miller supercharging and turbocharging
Here charge air is compressed more than the need of engine with the help of high pressure turbocharging and filling time is reduced by means of early closing of the inlet valve. The trapped fresh air charge further expands and cools down as the piston approaches BDC. When the temperature is lower at the beginning of the cycle, It leads to a lower mean temperature of the cycle and hence less thermal load and NOx production. In addition, increased air density and high air/fuel ratio at same cylinder pressure improves the SFOC and Power output of the Diesel Engine.
Exhaust gas recirculation
.Exhaust gas recirculation is a method of modifying the content of inlet air to reduce NOx emission. Here some of the exhaust gas is cooled and cleaned before recirculation to the Scavenge side its effect on NOx formation is partly due to the reduction of the oxygen concentration in the combustion and partly due to the content of water and carbon dioxide in the exhaust gas that increases the molar heat capacity of the mixed scavenge air and lower the combustion temperature which interns Curbs the formation of NOx. This particular system is very effective and more practical for the engines burning low Sulphur because gas engines operating on high sulphur fuel might invite corrosion of turbochargers intercoolers and scavenging pipes. Due to a higher specific mass flow through the cylinder, the lower material temperature is achieved.
Fuel injector nozzle modification
With the use of Slide valve introduce by MAN which eliminates the Sac volume of the injector and hence fuel after dripping from the nozzle. The main advantage of this slide type fuel valve is reduced emission of NOx, carbon monoxide, smoke and unburned hydrocarbon as well as significant decrease in deposits inside the engine. is reported that, by applying a slide type fuel valve to a container ship engine smoke reduction is observed nearly 40% and hydrocarbon and carbon monoxide be reduced by 33% and 42% and NOx was reduced by 14% while the fuel consumption remained Virtually unchanged.
Optimum fuel injection Timing, Retarded injection-
Electronically controlled injectors became the preferred means of achieving variable injection timing and offered unprecedented flexibility in injection time setting. Electronic control with common rail system gives high constant fuel pressure even at low load and precise control over timing which optimizes combustion and hence reduce temperature peaks and NOx reductions. However, this system individually has not proven desired emission reduction and must be used in conjugation with other NOx reduction methods for desired results.
Rate module injection and NOX optimize fuel spray pattern is also introduced by new electronically control engines which control the pattern of fuel injection. The engine can be run in echo and low NOx mode efficiently and changeover is possible even in running condition of the diesel engine.
- Secondary Method of NOx reduction- In this method, the produced Nox is removed from the exhaust gas by downstream cleaning techniques and up to 95% of NOX reduction has been claimed by this method.
- Selective catalytic reduction in this system the exhaust gas is mixed with ammonia preferably in the form of 40% solution of Urea in water and pass through a layer of special catalyst at a temperature nearly between 300 C and 450 C. The lower limit is determined by the sulphur content of the fuel at temperature below 270 degree Celsius Ammonia and SOx will react and deposits as Ammonium Sulphate where at excessive temperature the catalyst will be degraded. Urea dosing must be optimum in the different load condition for proper treatment along with the constant ammonia slip. Ammonia injection into the exhaust duct is controlled by the process computer as a function of engine load. With selective catalytic reduction NOX reduction, more than 90% can be achieved. The exhaust gas back pressure imposed by SCR plant is typically between 15 mili bar to 25 mili bars.
A typical compression of Humid air motor system (HAM) and SCR system Given by MAN
Particulates soot and smoke reduction.
Smoke is mainly formed either incomplete or partial combustion of fuel oil or when more fuel is injected with respect to available air(Normally during low load, during critical RPM and during Acceleration of diesel engine). Particulates matter are also formed by partial or unburned HC, Sulphate bound water, Sulphates, Ash and Elemental unburned/burned caron.
As per the study, nearly 50% to 70% of the particulate components are directly related to the quality of residual fuel oil and can be reduced just by changing over to cleaner oil. It can be further reduced by improvement of fuel oil and Lube oil quality, Exhaust gas cleaning and improvement of engine combustion process. Combustion process can be improved by combustion chamber geometry (Swirl, Penetration, and atomization) and Rate shaping by common rail technology.
With common rail fuel injection system both of these problems can be cured because it is always possible to maintain high injection pressure independent of the engine load, ensure good spray atomization and penetration even at very low load. The risk of over injection of fuel can also be avoided by a common rail system because it’s computer supplied with a temperature, pressure and rotational speed data. It can calculate the amount of air trapped in the cylinder based on the empirical map and the computer can decide how much fuel can be injected. In addition, it gives a faster load response than conventionally injected Engine.
@MAN B&W, WARTSILA